1. Field of the Invention
Aspects of the present invention relate to a polymer electrolyte membrane and a fuel cell using the same, and more particularly, to a polymer electrolyte membrane having excellent ionic conductivity at high temperatures, excellent mechanical characteristics, and thermal stability equivalent to that of a conventional poly(benzoxazole) polymer electrolyte membrane, and a fuel cell using the same.
2. Description of the Related Art
A fuel cell is a power generating system in which chemical energy produced in a chemical reaction between hydrogen and oxygen contained in a hydrocarbon material, such as methanol, ethanol, or a natural gas, is converted directly into electrical energy.
Fuel cells are categorized into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte membrane fuel cells (PEMFCs), alkali fuel cells, and the like. Although these fuel cells operate based on the same principle, the different types of fuel cells differ in terms of fuel, operation temperatures, catalysts, electrolytes, and the like.
PEMFCs have excellent energy generating characteristics in comparison to the other fuel cells mentioned above, and can operate at a low temperature, have a short start-up time, and respond quickly. A PEMFC can be used as, for example, a portable power source for cars, a distribution power source for homes and public places, or a small power source for electrical devices.
A PEMFC generally includes a polymer electrolyte membrane composed of a sulfonate perfluoro polymer (for example: NAFION obtained from Dupont Inc.) having a backbone of a fluorinated alkylene and a side chain of a sulfonic acid-terminated fluorinated vinyl ether. Such a polymer electrolyte membrane has excellent ionic conductivity when impregnated with a proper amount of water.
In a PEMFC including such a polymer electrolyte membrane, protons generated at an anode move to a cathode. When this happens, the protons are accompanied by water due to osmotic drag, so that the anode side of the polymer electrolyte membrane becomes dried, thus dramatically decreasing the proton conductivity of the polymer electrolyte membrane. Under these conditions, the PEMFC may fail to operate. In addition, when the operation temperature of the PEMFC is about 80° C. or greater, the polymer electrolyte membrane is further dried due to vaporization of water, and thus the proton conductivity of the polymer electrolyte membrane rapidly decreases.
Because of the tendency for the polymer electrolyte membrane to become dried at higher temperatures, a conventional PEMFC typically operates at 100° C. or less, for example at about 80° C. However, at such a low operation temperature of about 100° C. or less, other problems may result. For example, a hydrogen-rich gas, which is a fuel for the PEMFC, is obtained by modifying natural gas or an organic fuel, such as methanol. The hydrogen rich gas typically contains carbon dioxide and carbon monoxide as side products, and carbon monoxide can poison a catalyst contained in an anode. When this happens, the electrochemical activity of the poisoned catalyst is dramatically decreased, thus decreasing the operating efficiency and lifetime of the PEMFC. Such poisoning is more likely to occur as the operation temperature is decreased.
When the PEMFC operates at about 150° C. or greater, catalyst poisoning due to carbon monoxide can be prevented, the activity of the catalyst is increased, and the water management of the PEMFC can be more easily controlled. As a result, the volume of fuel reformer can be reduced and a cooling device can be simplified, and thus, the entire PEMFC energy generating system can be made smaller. However, as discussed above, when a conventional electrolyte membrane comprising a polymer electrolyte such as NAFION is used, the performance is substantially decreased due to the evaporation of water at high temperatures, and thus, operation of the PEMFC becomes almost impossible. Due to these problems, the need for a PEMFC that can operate at high temperatures has drawn much attention.
Many methods of manufacturing a PEMFC that can operate at high temperature have been developed.
For example, the use of polybenzimidazole (PBI) is disclosed in U.S. Pat. No. 5,525,436. This method is commonly used and has many advantages including operation at about 200° C. and thus substantially less catalyst poisoning due to carbon monoxide, excellent oxidation stability, and excellent thermal stability.
However, PBI has room for improvement in terms of mechanical strength, ionic conductivity, and the like. In particular, PBI is typically doped with phosphoric acid, and the doping level of the phosphoric acid is directly related to the ionic conductivity of the polymer electrolyte membrane. However, when the doping level of the phosphoric acid doped in PBI exceeds 800%, the PBI electrolyte membrane fails to retain its form, and thus the manufacturing of the PEMFC becomes complex and the ionic conductivity becomes limited.
In order to avoid the limitations of PBI, the use of poly(benzoxazole) in a PEMFC has been suggested. However, it is necessary to improve the impregnating procedure of poly(benzoxazole) because it is difficult to impregnate poly(benzoxazole) with a phosphoric acid using conventional methods.